Few months ago I was replacing several photocells on parking lot lights, a hail storm had broken them all.

The typical Mercury vapor "plug-in" style photocell is a thermal device. Line power is applied to the photocell and a resistor in series. The resistor heats a snap switch that is open as long as the photocell flows enough current. This is the "daytime" condition. At night the photocell is "off", no current through the resistor and the snap switch is closed.

The reason I was asking was because of something I saw today. Inexpensive florescent fixtures with photocells (small 'eye' on top) were installed all around a house last year. One fixture in the front recently stopped working and an exact replacement couldn't be found. It was decided that one from the back would come up to replace the one in the front and keep a matched set. All the lights were functioning fine to my knowledge.

When the one from the back was put in the front it would not shut off. The location in the back was unsheltered with the 'eye' pointing at open sky. The location in the front was now under an overhang (brown) therefore lightly shaded. The existing fixture about 4 feet away in same location was operating fine. I was wondering if it somehow could've gotten used to the lighter location over a years' time. Keep in mind that this is probably the cheapest form of photocontrol available.

Don't know, I'm just fishin' for possibles here. This is probably the least sophisticated method of photocontrol. It came with a fixture from a homecenter that cost (Utility rebate action here) $3.99 with a PL lamp included. It may have been exposed to direct sun where it was, if that makes a difference. ??

Maybe We should get a primer on how these things work?

Anybody know anything about innerworkings of various types of photocells?

Most of the photo cells that we in the trade would install [like the Intermatic "Ice Cube" ones that screw into a bell box] either use a PhotoDiode or a Photoresistor for the light sensing device. This would be the element directly behind the clear lens.On these, manufacturer specs say to aim it to the North - in order for proper light sensing. Even on these types, I have found many failures when they are directed towards the Sun [directed West]. On these, the lens is sunbleached.Almost every troubleshot photocell has been directed towards the West. The few that weren't had been damaged by some form of physical abuse [something or someone fell on it ].

They are rather simple devices and easy to understand. The Ambient light detection is done with either a PhotoDiode or a PhotoResistor. With these guys, having X amount of light falling across them will allow current to flow [in the PDiode], or will have a lower Resistance and higher current flow [in the PResistor]. Both would work on a small 1 pole relay, in which the line voltage control circuit would be connected across NC contacts.Time delay - in the form of an RC circuit [Resistor-Capacitor] is used to eliminate false triggering.

I would imagine [this is just a guess], that in Bill's situation, the light detection element was a simple PhotoResistor. Along with that, the lens was distorted from the UV light being directly applied to it at high levels, instead of being dulled from reflections. This, plus the possibility of the PResistor being kind of reduced from the high levels of UV light, would appear to have made the photocell only responsive to high and direct light levels.

BillMore than likely, the sun shining through the lens of the photo cell has created a high enough ambient temperature in the photo cell to distort the shape of the back plane of the photo resistor. This would change the shape of the resistor pack and also the resistance. The photo cell isn't remembering the light it just needs more to make it operate.

Multiple photo cell set ups on light systems are an easy place for an electrician to sell a photocell and lighting contactor. The customer will be verry pleased with the results.

I didn't mean to make it seem like I was sugesting that it was "remembering" the light somehow, only that it seemed like the sensitivity had changed somehow. I was just fishing for some possible scientific explaination for what seemed to have happened.